During the past fifty years the side chain chlorination of ethylbenzene has been studied with a number of different chlorinating agents including hypochlorites. Very good selectivity to alpha chlorination was observed, but only to monochlorination. There are no examples of a preparative method for the selective halogenation of alkylaromatics to compounds having at least two alpha halogen substituents, especially when beta hydrogens are present on the side chain.
Various references over the years have disclosed the chlorination of ethylbenzene using different chlorinating agents. The predominant product is .alpha.-chlorethylbenzene with some .beta.-chlorethylbenzene or .alpha.,.beta.-dichlorethylbenzene, but little or no .alpha.,.alpha.-dichloroethylbenzene. Representative references are J.Am.Chem. Soc. 61, 2142 (1939); Chem.Abst. 41:3437a (1947); Bull. soc.chim.Belges 59, 193(1950); J.Org.Chem. 28, 3173 (1963); J.Org.Chem. 39, 3472(1974) and J.Org.Chem. 44,2270(1979).
The chlorination of diethylbenzene in the presence of PC1.sub.5 and light showed poor selectivity to multichlorination in the alpha positions [Macromol. 6, 815(1973)]. Analysis of the product of the reaction of meta-diethylbenzene with chlorine and PC1.sub.5 conducted at 90.degree.-100.degree. C. with a 300w incandescent light lamp showed a reaction mixture containing "at least 25 different components of which 6-8 could be considered major constituents".
British patent No. 1,563,164 (1980) describes the preparation of .alpha.,.alpha.-dichloroethylaromatic compounds by a chlorination with molecular chlorine of the corresponding monochlorinated ethylaromatic compounds using a phosphorus halide catalyst, e.g. PCl.sub.3 or PCl.sub.5, in the presence of light or other initiator. Selectivities of 80-90% to the desired .alpha.,.alpha.-dichloroethylaromatic compounds are taught, but conversion of the ethylaromatic is only up to about 50%. A comparative example in which the same system was used starting with ethylbenzene gave a product mixture of 58.5% .alpha.-chloroethylbenzene, 31.5% .alpha., .alpha.-dichloroethylbenzene, 6.8% .alpha.,.beta.-dichloroethylbenzene and 3.2% higher chlorinated products.
In 1945 Prof. J. Kenner reported [Nature 156, 369(1945)] that his colleague Dr. R. F. Garwood had demonstrated that t-butyl hypochlorite in the presence of benzoylperoxide effectively chlorinated ethylbenzene to .alpha.-chloroethylbenzene. No experimental conditions, yields or the formation of any dichloro- products were mentioned.
In 1960 Walling initiated a series of studies on t-butyl hypochlorite as a chlorinating agent [J.A.C.S. 82, 6108(1960)]. These focused on kinetics and mechanism, and the reaction conditions involved large excesses of hydrocarbon over tert-butyl hypochlor-ite. In the case of the chlorination of alkylaromatics, little or no dichlorination was observed, and there is no mention of the selectivity to dichlorination.
U.S. Pat. No. 3,251,887 (1966) employs trichloromethanesulfonyl chloride in the presence of a free-radical generating catalyst to chlorinate ethylbenzene selectively to obtain (1-chlorethyl)benzene and states that in the case of ethylbenzene, tert-butyl hypochlorite gives no better selectivity to the alpha over the beta isomer in monochlorination than does the photochlorination with molecular chlorine.
Sodium hypochlorite (bleach) at a pH of 7.5-9 in the presence of phase transfer catalysts selectively chlorinates alkylaromatics in the alpha position [J.A.C.S. 105, 7672, (1983)]. For a 94% toluene conversion the product yields were: benzyl chloride (64%). benzal chloride (11%), with compounds such as benzoic acid, cresols, benzaldehyde, benzyl alcohol and ring chlorinated compounds making up the remainder. No attempt was made to optimize dichlorination and for the ethylbenzene reaction no product distribution was given.